In general, a lithium ion battery is used at a high drive voltage (0 to 5V). Therefore, when a lithium ion battery is exposed under a high temperature (40° C.) for a long time after being charged completely, self-discharge may occur due to a high voltage difference between a cathode and an anode. Additionally, a cathode has reactivity to a non-aqueous electrolyte so that decomposition may be generated and thus the capacity of a battery may decrease and the impedance of a battery may rapidly increase. This has been one of the most serious problems of lithium ion batteries.
To solve this, many attempts have been made to decrease the reactivity of an electrode to an electrolyte by adding a small amount of additives to an anode, an electrolyte or a cathode, or by appling a coating layer formed of inorganic materials or organic materials on the powder surface of a cathode or an anode. Additionally, Japanese Patent Laid-Open No. 98-255839 discloses that alkaline earth metal hydroxides are partially incorporated into a cathode active material to prevent reduction of battery capacity after storage at a high temperature.
However, there is no description with regard to the incorporation of any other metal hydroxides except for alkaline earth metal hydroxides in a cathode active material, for the purpose of improving storage properties of a battery at a high temperature.
Further, although metal hydroxides are good additives for improving storage properties of a battery at a high temperature, metal hydroxides are nonconductors by nature. Accordingly, if metal hydroxides are added to cathode active materials in an excessive amount, they may decrease battery capacity and degrade storage properties of a battery at a high temperature. Additionally, because metal hydroxides are not capable of lithium ion intercalation/deintercalation, increase of the amount of metal hydroxides added to a cathode of a battery results in decrease of the amount of a cathode active material that may be incorporated into the cathode, thereby causing decrease of battery capacity. Therefore, in order to minimize decrease of battery capacity, it is necessary to minimize the amount of metal hydroxides added to a cathode of a battery. However, it has remained undiscovered heretofore that the specific surface area of a metal hydroxide added to a cathode of a battery is related with storage properties of a battery at a high temperature, and that a small amount of metal hydroxide may be used to improve storage properties of a battery at a high temperature, based on such relationship.